This invention relates to a power tool operable at a variable driving speed and including a heat radiating block provided for a driving mechanism and a speed reduction mechanism which have improved structures.
In general, there is known a power tool such as an impact driver or an impact wrench for performing an automatic screw clamping operation. When it is required to clamp a screw by using such a power tool, the power tool is first driven at a low speed for positioning for the screw clamping and then driven at a high speed after the positioning for the screw clamping. In a case where the impact driver is utilized, the revolution numbers of a motor of the impact driver is changed by utilizing a power control element.
In such usage of the power control element, however, there causes a heat generation, which may result in heat breakage of the power control element itself, and in order to obviate such fear, in prior art, the power control element is attached to a heat radiating block thereby to escape the heat of the power control element to prevent the breakage thereof due to the generated heat.
In the structure of the power tool provided with the additionally incorporated heat radiating block, it is necessary to preliminarily set a space for locating the heat radiating block in the power tool, resulting in an enlargement of the power tool itself.
Furthermore, in a general power tool including a motor, a motor shaft is constructed so as to have a cantilevered beam structure, so that there is a fear of the motor shaft being deflected. In a case when such deflection is caused, mutual engagement of gears as a speed reduction mechanism disposed in the power tool is made worse and abnormal noises may be generated. In a certain case, abrasion may be caused between the gears, thus deteriorating the durability of the gears.
Moreover, in the general power tool, since the motor shaft is integrally formed with a carrier member constituting a power transmission mechanism, it is necessary to produce the shaft from a material, by cutting the same, having a diameter larger than an outer diameter of the carrier member or to produce the shaft by first carrying out a forging step with dimensions necessary for the carrier member and then cutting off unnecessary portion therefrom. Such working is troublesome for the manufacture of the shaft, requiring much time and labour in addition to a problem of bad yielding with respect to the material.
Still furthermore, the prior art provides a problem in an assembling process such that a pin is press fitted into pin holes of the carrier member, a planetary gear and a further carrier member, respectively, and such fitting requires an exact alignment of these pin holes, thus providing difficulty in the assembling of these members.